The present invention relates to an alarm system utilizing a bidirectional wired television system, such as, a bidirectional community antenna television system or CATV system.
Systems for providing an alarm service to a large number of subscribers from a remotely located central station are known in the art, as for example, polling systems of the type disclosed in U.S. Pat. No. 3,765,016 in which a plurality of subscribers are sequentially interrogated to answer as to the occurrence of an abnormal condition and contention systems of the type disclosed in U.S. Pat. No. 3,996,578 in which the subscribers each detecting the occurrence of an abnormal condition in its monitoring area sends an alarm signal to the central station, and many systems of these types have been put in practical use. However, a system which is capable of handling a relatively large number of subscribers inexpensively is very scarce. With the coaxial cable network of the CATV system, the coaxial cables are generally installed to branch off the community antenna to the respective subscribers. In the system disclosed in U.S. Pat. No. 3,765,016, the central station is connected to a transmission loop having a plurality of series connected subscribers and a normally closed line relay is connected in series with each subscriber in the loop, thus making it impossible to utilize the previously mentioned CATV network of the branched-off coaxial cables as such. Another disadvantage is that since the interrogating signals for polling are pulse code signals and since each subscriber is connected in series with the transmission line, the cycle time required for the polling is long so that if the number of subscribers is increased, the time interval from the time that each subscriber receives the interrogating signal until the subscriber receives the next interrogating signal is increased, thus making it impossible to make an early alarming.
On the other hand, in the contention system of the type disclosed in U.S. Pat. No. 3,996,578, different frequencies are assigned to the respective subscribers and another different frequencies are assigned to the respective groups each including a plurality of the subscribers so as to discriminate the subscriber which has sent an alarm in accordance with the values of the frequencies associated therewith, thus requiring a large number of frequency discriminating devices, such as, demodulators, tuners or band-pass filters. Another disadvantage is that particularly where an alarm signal includes a signal indicative of the type of abnormal condition, such as, fire, burglary or gas leakage and the type of abnormal condition must be discriminated at the central station, the assignment of frequencies tends to become more difficult as the number of subscribers is increased, thus setting a limit to the maximum number of subscribers which would permit effective discrimination of the terminals and types of abnormal condition with a limited frequency band. While this disadvantage involved in the discrimination of the types of abnormal condition can be overcome by polling the subscribers for every type of abnormal condition, the interrogating signals for polling must also contain signal components indicative of the types of abnormal condition, and the types of abnormal condition are generally discriminated in accordance with different frequency component, thus further increasing the frequency band required for the polling interrogating signals. As a result, the discrimination of a large number of subscribers and a plurality of types of abnormal condition in accordance with different frequency signals within the limited transmission band of the CATV cable network, sets by itself a limit.